1. Field of the Invention
The present invention relates to probe cards and more particularly, to a printed circuit board having a stepped structure for use in a probe card, and a probe card comprising the same. The present invention relates also to a method for clamping the probe card using the stepped printed circuit board.
2. Description of the Related Art
The function of an integrated circuit must be examined by means of a probe card before packaging, so that defective pieces can be removed, increasing the final yield rate.
A vertical probe card generally comprises a printed circuit board (hereinafter referred to ‘PCB’), an adapter substrate, and a probe head. The adapter substrate has one side joined to the PCB and electrically connected to the circuits of the PCB. The probe head is connected to the other side of the adapter substrate, allowing electric connection of the vertical probes of the probe head to the PCB. When in use, the PCB must be clamped by a testing machine to ensure stability of the probe card. However, the clamping size acceptable to the testing machine is limited. Therefore, the thickness of the PCB must be within the clamping size acceptable to the testing machine. Further, the adapter substrate is conventionally laminated layer by layer by the so-called built up process to increase its thickness. Because the thickness of each medium layer is limited to 30-60 μm and the adapter substrate has to be formed vias therein by laser ablation, the thickness of the adapter substrate is normally defined within 0.8-1.1 mm. The thickness limitation on the PCB and the adapter substrate and the length limitation on the vertical probes of the probe head may cause a contact failure between the vertical probes and the test sample after the testing machine has been adjusted to the bottom limit position. In this case, the test cannot be performed.
In order to eliminate the aforesaid problem, there are providers who join two PCBs or adapter substrates with solder balls by the reflow solder process, keeping the two PCBs or the adapter substrates electrically connected together. This method increases the total thickness of the probe card, however it also brings drawbacks. The first drawback is that the use of the solder balls to electrically connect the two PCBs or the adapter substrates causes a signal reflection and loss, resulting in a poor signal transmission quality. The second drawback is that the leveling between the two PCBs or the adapter substrates becomes poor after the reflow soldering process due to technical problem and manufacturing conditions, such as size difference of solder balls. The third drawback is that the bonding force of the solder balls after the reflow soldering process may vary with operating conditions, and the relatively thinner adapter substrate may be damaged or the solder balls may be forced out of place upon an impact. The fourth drawback is that the application of reflow soldering process relatively complicates the manufacturing process and increases the manufacturing cost.